I was also quite disappointed with the 12.9 inch 2017 screen. It slightly yellow, but mainly the contrast and colors are just too washed out compared to my other monitors. I expected the new Ipad to have much better colors than my old Ipad 3. Maybe the Ipad 3 has slightly less saturated colors than the 12.9 2017 model, but nowhere near the difference, I would expect. I think the contrast is worse, not better. It's not a horrible bad screen, but I think it should be much better.
This also happened to me when I bought the Ipad 3. Everywhere I looked they had yellow screens (opened 4-5 different ones in different stores). Then I ordered one from apple, and that one was much better. Now the panel lottery starts again?